In both radio and landline telephone systems, a user communicates by means of a handset that includes a speaker at one end that is placed close to the user's ear and a microphone at the other end that is held close to the user's mouth. This placement ensures that the audio from the speaker does not reach the microphone causing feedback problems. If the handset is taken away from the face, however, the speaker audio has a free path to the microphone. This is especially true when the handset is placed facedown on a surface. The surface acts as a reflector, reflecting the audio from the speaker to the microphone.
Government regulations restrict the amount of speaker audio to microphone coupling. Many countries have this type of regulation due to their cellular radiotelephone systems producing high sidetone levels. The higher sidetone levels would produce an irritating feedback howl if the speaker audio is coupled to the microphone.
One approach to solving this problem is a mechanical fix. The microphone is surrounded by a rubber ring that, when the handset is placed on a surface, seals the microphone from the speaker audio, thereby reducing speaker to microphone coupling.
This approach, however, creates its own problems. The mechanical designer must incorporate the rubber ring into the handset's design, limiting the designer to certain designs. Also, the rubber ring does not always seal correctly, allowing the speaker audio to reach the microphone. In addition, the rubber ring does not prevent feedback between the time the handset is lifted from the surface and brought to the user's face. Accordingly, there is a need for an improved method of suppressing feedback in radiotelephone equipment.